1. Field of the Invention
The present invention relates to a method of updating the clock of a radio station of a cellular radiotelephone system including an assistance server for improving the acquisition of satellite data.
2. Description of the Prior Art
In the field of mobile telephony, it is proving increasingly necessary to be able to locate mobile telephones.
To this end, it is known in the art to associate a cellular radiotelephony device, for example of the Global System for Mobile communications (GSM) mobile telephone type, with a radio navigation satellite system (RNSS) receiver such as a Global Positioning System (GPS), GLONASS or GALILEO type receiver, by means of which the mobile device picks up transmissions from satellites to determine its position. Thus in the event of a road traffic accident, for example, the mobile device can calculate and transmit its position.
The position of the device may be determined in the following manner: a plurality of satellites transmit continuously a time-stamped signal that is picked up by the receiver. If it is synchronized to the clock of the satellites, the receiver can then measure the propagation time of this signal and deduce therefrom the distance between it and a particular satellite. A receiver of the above kind can determine its position by triangulation using three satellites. Each propagation time measurement represents the radius of a sphere centered on a particular satellite, the receiver being situated on that sphere. With two distance measurements, the position of a receiver is within a circle formed by the intersection of two spheres. A simultaneous third measurement reduces the intersection to two points, one of which is at a great distance away in space and is easily ignored.
However, because it is not totally synchronous with the GPS, the clock of the receiver is affected by a clock bias ΔT. In fact, the atomic clocks of GPS satellites are very accurate but the accuracy of the more rudimentary GPS receiver is fatally lower. The clock bias ΔT is the time difference between the receiver clock and the satellite clock and may be as high as several seconds. It is reflected in an error in measuring the GPS signal propagation time and thereby an error c.ΔT in the satellite-receiver distance, where c is the speed of light. This error affects all distances measured by the receiver. As the distances are not perfect because they are affected by the time bias, they are called pseudodistances. The time bias is unknown a priori and must be determined.
There is therefore a fourth unknown, for which it is necessary to measure at least one additional distance, and therefore to use at least four satellites, in order to solve a system of four equations in four unknowns.
The signal sent by each satellite is a signal whose spectrum has been spread by a phase-modulated pseudorandom code; the GPS receiver must acquire this signal. The receiver generates locally a replica of the signal and then delays the start of this replica until it is superimposed on that coming from the satellite. The delay is determined by correlating the two signals. The value of this delay is the time taken by the signal to propagate from the satellite to the user. This kind of measurement demands extremely high accuracy (better than 100 nanoseconds). The time taken by the signal to travel this distance is of the order of 50 milliseconds. However, because the clock of the GPS receiver is never fully synchronized to that of the satellites, the receiver has to adjust its clock constantly by a process of successive approximations to arrive at the maximum correlation of the two signals. Acquisition of the signal therefore necessitates a time sweep by the receiver.
Furthermore, the signal transmitted by each satellite is transmitted at a known frequency of 1575.42 MHz. The Doppler shift of the satellite, on which is superimposed a receiver local clock uncertainty, results in an uncertainty of ±5 kHz in respect of the signal received by the GPS receiver. Now, to obtain a good correlation, the signal generated locally by the receiver must have the same frequency as the signal transmitted by the satellite. The receiver must therefore perform a frequency sweep, in addition to the time sweep, in order to determine the time taken by the signal to propagate from the satellite to the user.
The time and frequency sweeps referred to above imply a very long data processing time and require a receiver having a very high computation power.
One solution is to use a server to assist the GPS receiver of the mobile device by increasing its sensitivity by curtailing the time-frequency area to be swept. A server of this kind is described in “Indoor GPS Technology” (F. van Diggelen et al., CTIA Wireless-Agenda, Dallas, May 2001). This technology is known as the assisted GPS (A-GPS) technology. FIG. 1 represents a telecommunication system 1 using an assistance server 5 of this kind. A mobile device 2 including a GPS receiver, such as a mobile telephone in a GSM type telephone network 4, is seeking to calculate its position from signals P1 to P4 transmitted by at least one of four satellites S1 to S4. To this end, the device 2 sends a request R in the form of a radio signal over the telephone network 4. The request R passes through a base transceiver station (BTS) type radio base station 3 associated with the cell in which the mobile device 2 is located. The request R is processed by the server 5, which receives satellite information in real time via fixed radio stations 6 equipped with GPS receivers receiving information K. In response to the request R, the server 5 sends to the mobile device 2 information I that passes through the BTS 3. The information contains, for example, the ephemerides of the satellites S1 to S4. Using that information, the mobile device 2 can determine the Doppler shift of the satellites and considerably curtail its frequency sweep. Note that there are two types of A-GPS technology, namely mobile station based (MS-based) and mobile station assisted (MS-assisted). In the case of the MS-based technology, the position of the mobile device 2 is calculated by the mobile device itself. In the case of the MS-assisted technology, the position of the mobile device 2 is calculated by the server 5.
In the synchronous code division multiple access (CDMA (or CMA 2000)) telephone networks used in the USA, the time sweep is much less critical since each of the transceivers is synchronized. Thus using an assistance server with a synchronous network greatly curtails the time and frequency sweep. On the other hand, in the case of an asynchronous network such as a GSM or UMTS network using time division multiple access (TDMA), the time sweep aspect remains very important even if an assistance server is used, because the transceiver clocks may vary considerably.
One prior art solution to this problem is described in the document “Analysis of GPS time-transfer accuracy in GSM and UMTS networks and possibilities to improve sensitivity” (J. Syrjärinne, ION GPS 2002, 24-27 Sep. 2002) and consists in equipping each base transceiver station (BTS) 3 with a GPS receiver, as shown in FIG. 1. The base station 3 can therefore, after calculating its position and the clock bias between itself and the satellites, determine the common clock of the satellites and transmit that GPS clock to the mobile device 2 by marking the information signal 1. The clock transmitted to the mobile device 2 is fairly accurate in that it is affected only by the transmission time of a few microseconds between the base station 3 and the mobile device 2. This transmission time is short because the mobile device 2 is in the cell associated with the base station 3 and the diameter of a cell such as a GSM cell varies from 300 m to 30 km, depending on terrain and population density.
However, implementing a solution of the above kind causes certain difficulties. Thus this solution is extremely costly, because it entails many modifications of the network and installing a GPS receiver in each BTS type radio base station.
The present invention aims to provide a method of updating the clock bias between the common clocks of the satellites of a radio navigation satellite system and the clock of a BTS type radio station of an asynchronous cellular radiotelephone network such as a GSM network, said method enabling mobile devices in said cellular network equipped with an radio navigation satellite system receiver to access this clock bias in a simple and economical way.